Invasive marine alien species are organisms that are non-native to an ecosystem, and which may cause economic or environmental harm or adversely affect human health. In particular, they impact adversely upon biodiversity, including decline or elimination of native species – through competition, predation, or transmission of pathogens – and the disruption of local ecosystems and ecosystem functions.

Alien species are also known as exotic, introduced, non-indigenous, or invasive species. As the names imply, these species do not belong to ecosystems in which they are either intentionally or unintentionally placed. They tend to disrupt the ecosystem’s balance by multiplying rapidly. These species are often plants, fishes, mollusks, crustaceans, algae, bacteria or viruses.

A great change in the Mediterranean fauna occurred after the opening of the Suez Canal in 1869, which connected the Red Sea to the Mediterranean Sea. The Suez Canal connects two major water bodies, the Red Sea and the Mediterranean, which differ fundamentally, both faunistically and hydrographically. The Canal, which is 162.5 km in length, 200–300 m in width and 10–15 m in depth, crosses Lake Timsah and the Bitter Lakes on its way to the city of Suez and the Gulf of Suez. The fauna of the Red Sea is of tropical Indo-Pacific origin, while that of the Mediterranean is mainly of temperate Atlantic origin. The Red Sea and Mediterranean Sea were exposed to invasions of organisms from each other, known as the “lessepsian migration”, named after Ferdinand de Lesseps, the French diplomat in charge of the canal construction.

In addition to the lessepsians, many other alien species are transferred into marine ecosystems of the Mediterranean through aquaculture peruses, fishing equipment, aquarium purposes, scientific purposes, fouling ship hulls, ballast water by commercial shipping operations.

Invasive alien species, introduced and/or spread outside their natural habitats, have affected native biodiversity in almost every ecosystem type on earth and are one of the greatest threats to biodiversity.

The Mediterranean Sea has been under tropicalization, and its warming is the one of the reason of the increase in the number of alien species in the Mediterranean. In recent years, there is increasing number of lessepsian invasion from Indo-Pacific and Red Sea which results in significant change of biodiversity in the Mediterranean. Some of endemic species is getting replaced with lessepsian species. Lessepsian species pressure on the Mediterranean species resulted in declining native species, and lessepsian species become more common in these areas by establishing their diet which also tend to increase the reproductive ability. The impacts of lessepsian species on their new environment include; restructuring established food webs, competition with native organisms for food and space and altering the gene pool when the invading organisms reproduce with native species, altering evolutionary processes and causing dramatic changes in native populations.

In addition, there are harmful effects of the alien species. These can be categorized as: (a) health problem (poisoning, pain) for fishermen, swimmers, divers, tourism, (b) net damages, (c) mesh clogging, (d) fouling and (e) extra labor for fishermen. There are three types of consequences of introduction of lessepsian immigrants in Turkey: ecological consequences, economic consequences, economic and ecological consequences